The present invention relates to a method for determining the fundamental oscillation frequency in an optical fibre during the drawing thereof from an optical preform, the oscillation of the optical fibre being measured perpendicular to the longitudinal axis thereof in order to form a frequency spectrum using Fast Fourier Transformation (FFT), from which the fundamental oscillation frequency is determined. The present invention also relates to a specific application of a fundamental oscillation frequency measured using such a method, which fundamental oscillation frequency can be converted into the tensile force.
The method referred to in the introduction is known per se from U.S. Pat. No. 4,692,615, the tensile force in a fibre being determined during the drawing process by determining the fundamental oscillation frequency by means of Fast Fourier Transformation. From this document it is known that the vibration behaviour of an optical fibre during the drawing corresponds to the vibration or oscillation behaviour of a string that is under tension fixed at both ends. According to the method known therefrom it has been found that the transverse movement of an optical fibre during the drawing process can be divided into a number of harmonic partitions whose frequency is related to the tension in the fibre. The harmonic analysis of the transverse vibration movement of the optical fibre is thus used to determine the tensile force, with the drawing rate and the tensile force at a constant furnace temperature being linearly related to one another. The accuracy of the frequency method for monitoring the tensile force was thus further increased by applying a linear regression model to the measured values. However, the tensile force that is determined via such a method proves to show substantial spread in practice, to which it can be added that the fundamental oscillation frequency cannot be accurately determined from the FFT spectrum thus obtained.
A method for determining the fundamental oscillation frequency using FFT is also known from U.S. Pat. No. 5,079,433, according to which an extra check is applied to the second harmonic oscillation frequency. A drawback of such a second harmonic oscillation frequency is that, in practice, it has a peak level that lies several orders below the fundamental oscillation frequency and it is difficult to determine it in the frequency spectrum.
In the manufacture of glass fibres for tele- or data communication a preform is with heating thereof drawn into a glass fibre with a diameter of 125 μm. In this manufacturing step the preform is slowly introduced into a furnace, the preform being heated to around 2000° C. In the furnace the preform melts and is drawn into a glass fibre, which leaves the furnace at the other end. The formed glass fibre is cooled, provided with a protective coating and wound onto a reel. The tensile force with which the fibre is drawn from the hot furnace is an important parameter, which partly determines the strength and optical properties of the fibre.
Before the glass fibre has been provided with a protective coating it is very susceptible to damage. Damage weakens the glass fibre, with the risk of it fracturing, which is undesirable. The concentration of dust in the area through which the unprotected glass fibre passes is therefore reduced by means of known clean-room techniques. Nevertheless, all optical fibres deriving from one preform or large parts of the optical fibre from one preform are sometimes too weak for further processing.
Microscopic analysis of such an optical fibre has shown that this was often caused by minute scratches on the optical fibre. It is believed that this is caused by the unprotected optical fibre touching a component in the drawing tower, or a glass splinter, or a small glass fibre particle, for example in a cooling tube. In spite of precautionary measures such as visual inspection or the cleaning of cooling tubes with brushes prior to the drawing process, weak fibres are sometimes produced.